Chemical sensors are highly useful in the chemical and pharmaceutical industry. Chemical sensors are used to determine the concentration in biological samples of metal ions, such as calcium, and important gases, such as NO, O2, and CO2. For example, sensors are used to determine the level of calcium ions (Ca(II)) in the blood of animals. In addition, chemical sensors are important in the development of analytical techniques to measure the enantiomeric purity of chiral compounds. Analytical techniques to determine enantiomeric purity are important to the pharmaceutical industry because many biologically active compounds are single enantiomers.
In general, a chemical sensor consists of a receptor and a transducer. The receptor selectively binds the analyte while the transducer generates a signal based on the change in a chemical parameter that occurs in response to analyte binding. The sensitivity of the sensor is defined by the minimum amount of analyte that induces an identifiable signal relative to the noise. The selectivity of the sensor corresponds to its capacity to distinguish the analyte from the other chemical species which may be present in the medium. A very selective sensor is distinguished by the fact that the signal induced by the presence of the analyte is much more intense than the signal induced by any other chemical species present in the same amount. One of the main difficulties encountered in this field is to prepare sensors which are both sensitive and highly selective.
Recently, there has been an increasing interest in the development of practical, sensitive and miniaturzed probe systems for use in monitoring metal ions in biological media, living cells, and environmental samples. Fluorescence based sensors are especially useful due to their sensitivity, achieving attomole (10−8 mole) detection limits. Such sensors often use probes that have chemical reagents or bioreceptors (such as an antibody) chemically bound to optical fibers or physically entrapped in sensing microcavities containing liquid reagents or gels attached to the distal end or to the cladding of the optical fiber. Direct attachment allows fast response time since sensor response depends on the mass transfer rate of the analyte to the immobilized reagent. In some cases, gels may be saturated with large quantities of reagent in order to enhance the sensitivity of the sensor. Physical entrapment onto the probe can also be another form of immobilization that is suitable to chemical or biological reagents. Immobilization on cellulose or poly(vinyl chloride) films allows greater loading, but decreases response time because the reagent is immobilized in single layers.
Several chemical sensors have been developed for the detection of metal ions. One of the first fluorescent calcium indicators was 2-{[2-[bis(carboxymethyl)amino]-5′-methylphenoxy]methyl}-6-methoxy-8-[bis(carboxymethyl)amino]quinoline (Quin-2). See R. Y. Tsien Biochemistry 1980, 19, 2396. Calcium binding to this sensor elicits an increase of fluorescence intensity of this compound. In comparison, Mauze et al. describe a chemical sensor comprising 2,2-bis(3,4-(15-crown-5)-2-nitrophenylcarbamoxymethyl)tetradecanol-14 which has at least one binding site and is provided with a fluorophore such as Rhodamine-B at that binding site. See U.S. Pat. No. 5,154,890. The sensing material is immobilized in a gel of polyacrylamide. U.S. Pat. No. 5,176,882 teaches a dual fiberoptic cell for multiple serum measurements where both a gas and an ion are analyzed simultaneously using a single probe having two separate fiber optic sensors incorporated therein. The gas is detected by the color change of a dye and the ion is detected by the fluorescing of a fluorescent metal ion sensitive dye. Nevertheless, sensors which have been developed to this point have several deficiencies, including low sensitivity due to weak analytical signal, low selectivity due to interferences, long term instability because of degradation of the immobilized reagent or its desorptive loss from the support, and/or slow response time because of barriers to mass transport in the polymer support.
Analytical methods to measure enantiomeric purity are needed because many biologically active compounds, such as pharmaceuticals, agrochemicals, flavors, and nutrients, are chiral. In fact, more than 50% of today's top-selling drugs are single enantiomers. The increasing demand for enantiopure chemicals has been accompanied by significant progress in asymmetric synthesis and catalysis. See (a) Helmchen, G.; Hoffmann, R. W.; Mulzer, J.; Schaumann, E. (Eds.) “Stereoselective Synthesis” in “Methods of Organic Chemistry”, Houben-Weyl, Vol. 21a-21f, 4th edition, Thieme, Stuttgart, 1995; (b) Gawley, R. E. Aubé, J. “Principles of Asymmetric Synthesis” Tetrahedron Organic Chemistry Series, Elsevier, N.Y., 1996; (c) Ho, T.-L. “Stereoselectivity in Synthesis” Wiley-VCH, New York, 1999; (d) Jonathan, M. J. W. “Catalysis in Asymmetric Synthesis” Sheffield Academic Press, Sheffield, 1999; and (e) Ojima, I.; (Ed.) “Catalytic Asymmetric Synthesis” 2nd edition, Wiley-VCH, New York, 2000. In addition, many analytical techniques, such as chiroptical methods, NMR spectroscopy, mass spectrometry, electrophoresis, and chromatography using chiral stationary phases, have been developed for the determination of the enantiomeric purity of chiral compounds. See Eliel, E. L.; Wilen, S. H. “Stereochemistry of Organic Compounds” John Wiley and Sons, New York, 1994, pp. 214-274.
Stereoselective analysis is very important to verify the purity and stereochemical stability of chiral chemicals and drugs. It also plays an integral part in the development of new asymmetric reactions. Recently, high-throughput screening (HTS) methods based on chiral chromatography for fast evaluation of enantioselective catalysts have been developed. See Wolf, C.; Hawes, P. A. J. Org. Chem. 2002, 67, 2727-2729; Wolf, C.; Francis, C. J.; Hawes, P. A.; Shah, M. Tetrahedron: Asymm. 2002, 13, 1733-1741; and Duursma, A.; Minnaard, A. J.; Fering a, B. L. Tetrahedron 2002, 58, 5773-5778. It has been demonstrated that multi-substrate screening followed by chromatography can provide yields, stereoselectivity, catalytic activity, chiral induction, and substrate tolerance of a catalyst in a single experiment. However, employing chromatography in HTS is usually too time-consuming, i.e. individual substrate screening combined with real-time enantioselective analysis seems to be a more promising approach.
Routine analysis of the enantiomeric composition of a sample usually entails chiral chromatography using expensive GC or HPLC columns, chiroptical methods, electrophoresis with chiral additives or NMR spectroscopy with chiral shift reagents. Enantioselective sensing based on fluorescence spectroscopy offers a variety of advantages over these techniques including different detection modes (fluorescence quenching, enhancement, and lifetime), high sensitivity, low cost of instrumentation, waste reduction, time efficiency, and the possibility of performing real-time analysis. Because of the high sensitivity inherent to fluorescence spectroscopy only a very small amount of the sensor is required, which makes this technique more cost-effective and practicable than chromatography. To date, only a few enantioselective fluorescence sensors including chiral macrocycles, dendrimers, or oligomers have been reported. See Lin, J.; Hu, Q.-S.; Xu, M.-H.; Pu, L. J. Am. Chem. Soc. 2002, 124, 2088-2089; Lee, S. J.; Lin, W. J. Am. Chem. Soc. 2002, 124, 4554-4555; Xu, M.-H.; Lin, J.; Hu, Q.-S.; Pu, L. J. Am. Chem. Soc. 2002, 124, 14239-14246; and Ma, L.; White, P. S.; Lin, W. J. Org. Chem. 2002, 67, 7577-7586.
Enantioselectivity in energy transfer reactions between a variety of chiral quencher molecules and photoexcited chiral Lanthamide chelates has also been observed by time-resolved or steady-state circularly polarized luminescence measurements. See Meskers, S. C. J.; Dekkers, H. P. J. M. J. Am. Chem. Soc. 1998, 120, 6413-6414 and Meskers, S. C. J.; Dekkers, P. J. M. J. Phys. Chem. A 2001, 105, 4589-4599. Nevertheless, the development of enantioselective sensing to a broadly applicable technique requires the design of new selector molecules combining well-defined stereoselective recognition of various classes of compounds with the advantage of fluorescence detection.